The propulsion unit, whether orientable or not, can be disposed at the lower end of a housing containing the motor unit, this housing sliding vertically upwards in a well inside the boat in order to retract said propulsion unit.
For high-power propulsion units, in which the dimensions of the housing and of its well in the boat are large, appreciable deformation occurs on the walls of said well which make it necessary to leave a relatively large clearance between the housing and its well in order to allow it to slide.
This causes poor lateral stability of the housing in its well and a fairly large leakage of water between the walls of said housing and of the well, whence there is a danger of hindering the access of personnel to the housing.
Preferred embodiments of the present invention mitigate these disadvantages by clamping the sliding housing either in the retracted position which reduces the draft of the boat, or in the extended position which provides sealing between the housing and its well to allow access of personnel to the housing.